METHOD OF CONTROLLING CHARGE DOPING IN VAN DER WAALS HETEROSTRUCTURES
The present disclosure is directed to controlling charge transfer in 2D materials. A charge-transfer controlled 2D device comprises a 2D active conducting material, a 2D charge transfer source material, and at least one overlapping portion wherein the 2D active conducting material overlaps the 2D charge transfer source material including at least one edge of the 2D charge transfer source material.
This application claims the benefit of priority to U.S. Provisional Patent Application No. 63/203,191 filed Jul. 12, 2021, which is incorporated by reference herein in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENTThis invention was made with government support under DMR-1810305 and DMR-2003343 awarded by the National Science Foundation and N00014-20-1-2308 awarded by the Office of Naval Research. The government has certain rights in the invention.
FIELD OF THE DISCLOSUREThe field of the disclosure relates generally to using one 2D/layered crystalline material to donate/remove electrons from another 2D/layered crystalline material. The field of the disclosure relates specifically to novel methods of locally adding electrical charge to atomically thin (2D) materials, and more specifically to adding local electrical charge by layering flakes of a charge transfer material in atomically thin graphene devices, such as by layering flakes of α-RuCl3 on top of an atomically thin graphene device. Advantages of this novel method include generation of atomically sharp p-n junctions.
BACKGROUND OF THE DISCLOSUREModulation doping in crystalline films produces extreme carrier mobilities for fast/high power electronics, efficient optoelectronics, qubits, the fractional quantum Hall effect, and topological superconductivity. However, two-dimensional (2d) van der Waals materials lack crystalline dopants for permanent, large, uniform, and local control of charge densities. Previous attempts utilized ionic liquid and polymer electrolyte gating, atomic/molecular intercalation, functionalization, and adsorption. Densities exceeding 1014 cm−2 were achieved in graphene, though at significant cost to sample quality. Furthermore, these chemical approaches cannot be applied to air sensitive materials nor specific layers of the heterostructure.
Accordingly, there is a need for local addition of electric charge to atomically thin graphene devices and to gain control of the flow of electrical current through atomically thin materials for applications including but not limited to photovoltaics and computing. As shown herein, these limitations are circumvented with an insulating two-dimensional material that acts as a crystalline acceptor.
BRIEF DESCRIPTION OF THE DISCLOSUREIn one aspect, the present disclosure is directed to a charge-transfer controlled 2D device comprising a 2D active conducting material, a 2D charge transfer source material, and at least one overlapping portion wherein the 2D active conducting material overlaps the 2D charge transfer source material including at least one edge of the 2D charge transfer source material.
In another aspect, the present disclosure is directed to a method for controlling charge transfer in 2D materials, comprising providing a 2D active conducting material, providing a 2D charge transfer source material, and positioning the 2D active conducting material to overlap at least one portion of the 2D charge transfer source material including at least one edge of the 2D charge transfer source material.
In yet another aspect, the present disclosure is directed to a charge-transfer controlled 2D system comprising a top gate, a charge-transfer controlled 2D device, and a bottom gate. The charge-transfer controlled 2D device includes a 2D active conducting material, a 2D charge transfer source material, and at least one overlapping portion wherein the 2D active conducting material overlaps the 2D charge transfer source material including at least one edge of the 2D charge transfer source material.
In some aspects, the at least one overlapping portion includes the 2D active conducting material being in direct contact with the 2D charge transfer source material. In some aspects, the device further comprises an insulating layer such that the at least one overlapping portion comprises the insulating layer disposed between the 2D active conducting material and the 2D charge transfer source material. In some aspects, the 2D active conducting material is selected from graphene, WSe2, and EuS. In some aspects, the 2D charge transfer source material is a crystalline and/or an exfoliated material. In some embodiments, the 2D charge transfer source material is selected from alpha-RuCl3 and tungsten oxyselenide. In some aspects, the insulating layer is selected from hexagonal boron nitride and AlOx. In some aspects, the top gate is selected from Cr/Au and hexagonal boron nitride-supported Cr/Au and the bottom gate includes SiO2/p-Si.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The embodiments described herein may be better understood by referring to the following description in conjunction with the accompanying drawings.
Disclosed herein are new methods to controllably charge dope semiconductors and semimetals that exist in single- or few-layer flake forms. There is presently enormous interest in the field broadly known as “atomically thin materials” or “2D materials” which grew out of the discovery of graphene. Much activity focuses on the custom assembly of quasi-3D devices made by stacking single- and few-layer-flakes of materials chosen for their particular properties, in order to end up with a device with novel functionality. Such structures are commonly known as “van der Waals heterostructures.” In some embodiments, flakes of α-RuCl3 (which in bulk is a weak semiconductor at room temperature) are incorporated next to graphene, a large irreversible charge transfer occurs, with electrons moving from graphene to α-RuCl3. The effect persists in a range of other 2D active conducting materials including but not limited to bilayer graphene, WSe2 (a layered semiconductor), EuS (a magnetic semiconductor), and other atomically thin conducting materials. The effect yet persists when a thin insulating film is placed between the α-RuCl3 and graphene, and in fact this embodiment operates as a method to control the magnitude of the charge transfer.
Also disclosed herein are ultra-sharp (≤10 nm) lateral p-n junctions in graphene using a combination of electronic transport, scanning tunneling microscopy, and first principles calculations. The p-n junction lies at the boundary between two differentially-doped regions of a monolayer graphene sheet, where one side is intrinsic and the other is charge-doped by close proximity to a flake of α-RuCl3 across a thin insulating barrier. The p-n junction contribution to the device resistance is extracted, and used it to place bounds on the junction width. Whether the junction is ultra-sharp or not is correlated with the presence of a straight cleaved edge of the dopant α-RuCl3 flake. That is, ultra-sharp junctions require the cleaved straight edge. Scanning tunneling spectroscopy in heterostructures of graphene, hexagonal boron nitride, and α-RuCl3 shows potential variations on a sub-10 nm length scale. Density functional theory calculations reveal the charge-doping of graphene decays sharply over just nanometers away from the edge of the α-RuCl3 flake.
Charge Doping Control in Van Der Waals Heterostructures
As described herein, doping is defined as adding impurities to a material that either donate or remove electrons from a semiconductor. “p” and “n” relate to removing or adding electrons. In materials, removing electrons leaves behind “holes”, which behave exactly like electrons in that they can constitute a flow of electrical current; but they are positively charged, rather than negatively. p=positive, n=negative, describing the sign of the charge transferred. A p-n junction is at the heart of modern technology including diodes (current flows only one way), light-emitting diodes (e.g., modern computer screens), photovoltaics, and also transistors in every microchip. The primary issue is to have spatial control over the charge doping of the semiconductor. Moreover, the semiconductor industry needs new technology for when Si transistor minimum size is reached. One way forward is atomically-thin materials, such as but not limited to graphene, WSe2, and EuS. Until the present disclosure, it was unknown how to controllably create a permanent charge doping (see
New methods and new devices are disclosed herein. Making p-n junctions was the key technological breakthrough underlying transistor technology and hence microchips, computers, and the entire digital/hardware/software, and infrastructure built on that technology. This advance was the development of an ability to controllably charge-dope (meaning, to add or remove electrons) silicon and other semiconductors, which enabled the transistor-based technological revolution. The present disclosure provides the basis for recapitulating this advance in the novel field of atomically-thin materials. Accordingly, disclosed herein is for a technique to remove electrons from the target materials, including while still creating p-n junctions using electrostatic gating, which is a key technology not available in bulk semiconductors (like silicon) but which could theoretically work in 2D materials. Thus, the present disclosure achieved a laboratory demonstration of a p-n junction in graphene. While p-n junctions have been made previously, defining the two charge regions solely by electrostatic gating, this has disadvantages including impermanence of charge (it disappears or varies with the applied gate voltages), and the p-n junction itself is quite broad, and therefore not very technologically useful. As described herein, the p-n junction can be made almost atomically sharp, and data is consistent with such ultra-sharp p-n junctions. The ability to make p-n junctions, and to make them ultra-sharp, has great commercial relevance. The sharper the p-n junction, the stronger the in-plane electric field at the junction, which is critical for charge generation in photovoltaic and light-detection applications. It can also enable novel “electro-optic” applications by making “negative refraction” possible, extending many forefront optical ideas into the electronic realm. It enables precise control over the modulation of charge doping in graphene and other semiconductors, so that transistor-like elements can be readily defined with just a few layers of 2d materials stacked together as disclosed herein; and therefore may finally enable computing devices to be made of atomically thin materials.
In contrast to previous devices, the devices described herein comprise an active conducting layer/region that remains chemically identical the entire time, and by placing other materials nearby, a spatially localized/patterned charge transfer is effectively created without changing the chemical makeup of the active layer. Further, two regions are created in the same material with different charge states. In this way, the devices and methods described herein achieve a doped semiconductor material entirely of 2D materials.
In some embodiments herein, materials (such as alpha-RuCl3) are used to remove/accept electrons in atomically thin 2D devices. Depending on the embodiment, the actively conducting region/layer (also called a target material) of the device is selected from graphene, WSe2, EuS, and suitable alternatives such that two regions of separate charge are defined in the material and creates a p-n junction. In embodiments comprising graphene, additional application of an electrostatic gate voltage enables additional control over charge doping in either/both sides of the device. In some embodiments, the active layer/region comprises a material having a band gap or a semiconducting layered material which can be made in the atomically-thin limit. In embodiments comprising WSe2 that is natively n-type, the charge transfer source (e.g., alpha-RuCl3) creates a p-type region next to the WSe2 making a monolithic p-n junction from solely these two materials. In other embodiments, materials are used that can donate electrons in order to give full control over device charge-doping profiles. In yet other embodiments, monolithic p-n junctions are created from three materials: one is the active region that currents flow through, the other two donate or remove electrons to change the charge state.
van der Waals heterostructures made by custom-stacking atomically thin materials together typically control charge by applying electric fields to the devices. In contrast, and as disclosed herein, adding layers of a material that serves as a source of charge transfer (including but not limited to alpha-RuCl3, oxidized layers of WSe2 known as tungsten oxyselenide, and other 2D charge transfer source materials) essentially soaks up a fixed amount of electrons, enabling permanent charge transfers that do not require the external electric field.
In exemplary embodiments described herein, alpha-RuCl3 (known for its antiferromagnetic properties for quantum spin liquids) is readily able to transfer charge to several different types of materials including, though not limited to, graphene. Adding charge to a quantum spin liquid is one mechanism thought to underlie the physics of high-temperature superconductivity. In some embodiments, placement of a single layer of alpha-RuCl3 on top of an atomically thin material/device successfully and sufficiently created and transferred charge. In some embodiments, placement of a thin sheet of an electrically insulating material between alpha-RuCl3 and graphene is not detrimental to charge transfer within the device. In these embodiments, varying the thickness of the insulator controls how much charge flows in. Accordingly, the methods described herein effectively enable modulation doping (i.e., physical and spatial separation of the source of charge from where it goes). In typical heterostructures, atoms must be added to bulk materials, which causes lots of disorder. However, as disclosed herein, layering a source of charge transfer enables charge to flow right in, and there is no need to change the chemical structure of the device with bulk materials, thus making the methods described herein a ‘clean’ way to add charge.
Two-dimensional nanoelectronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. As described herein, Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations that establish the large work function and narrow bands of α-RuCl3 enable modulation doping of exfoliated (e.g. flaked) single and bilayer graphene, chemical vapor deposition grown graphene, WSe2, molecular beam epitaxy grown EuS, and other 2D active conducting materials. Proof of principle is further demonstrated for photovoltage devices, control via twist angle, and charge transfer through hexagonal boron nitride. Short-ranged lateral doping (≤65 nm) and high homogeneity are achieved in proximate materials with a single layer of α-RuCl3. This leads to the best-reported monolayer graphene mobilities (4900 cm2/(V s)) at these high hole densities (3×10′3 cm−2) and yields larger charge transfer to bilayer graphene (6×10′3 cm−2).
Exemplary embodiments of the present disclosure focus on α-ruthenium(III) chloride (α-RuCl3), a van der Waals, narrow-band Mott insulator with a deep work function of 6.1 eV (
To establish α-RuCl3 as a crystalline acceptor and bring modulation doping to two-dimensional crystals, spatially resolved Raman spectroscopy was employed. This allows rapid probing of the induced charge, strain, homogeneity, lateral, and vertical extent of the charge transfer in a variety of α-RuCl3 heterostructures without fabrication. Results disclosed herein provide the first unambiguous evidence that even a single layer of α-RuCl3 is able to strongly charge the target layer even when hBN is between them, including higher doping in bilayer graphene. A variety of proof of principle experiments further point to its utility: creation of a p-p′ homojunction for 2D optical sensors and electronics, and charge transfer to chemical vapor deposition-grown (CVD) graphene and WSe2, as well as molecular beam epitaxy-grown EuS (see
Recently, electronic transport experiments and first-principles calculations also suggested α-RuCl3 can dope monolayer graphene (mlg) to hole densities of a few 1013 cm−2. However, the two experiments also showed Dirac points close to zero gate voltage. Furthermore, the Hall and quantum oscillation data imply multiple carrier densities or a splitting of the Dirac cone. As shown through careful Raman studies, these features resulted from regions where the two materials do not touch. Indeed, since transport averages over the whole device it will include contributions from both the nearly charge neutral and strongly hole-doped regimes. Creating uniformly doped samples is crucial for eventual device functionality and, according to electronic structure calculations, will strongly affect the electronic properties of the combined system. Beyond disorder, the lateral and vertical extent of the charge transfer, dependence on layer number and relative rotation, ability to charge dope materials beyond mlg, and prototypical devices remain unexplored.
Device (D1) is a single monolayer graphene sheet laid across both mono- and bilayer α-RuCl3, all supported by a SiO2/Si substrate. This and the other structures designed, constructed, and characterized herein represent a new class of devices, incorporating α-RuCl3- or hBN-supported graphene that either lack contacts or have etched contacts at the graphene edge. This ensures that the interface between graphene and α-RuCl3 is not affected by the presence of metallic leads. The room-temperature Raman spectra is shown in
The doping and strain corresponding to the G and 2D peak shifts are determined following procedures described herein below. In
To determine whether this charge transfer capability is unique to α-RuCl3 or is generic to all layered halides, devices were investigated incorporating CrCl3, a magnetic semiconductor with a similar lattice structure to α-RuCl3. Density functional theory (DFT) calculations show the conduction band of CrCl3 is quite close to the Dirac point of graphene (
Next the thickness dependence of the charge transfer between α-RuCl3 and graphene layers was studied. First the spatially resolved map of the Raman G peak frequency for device D1 was studied, as shown in
The same is not true for graphene, where it was found that bilayer is more heavily doped than mlg. Specifically, a heterostructure device (D2) was measured having contiguous mono- and bilayer graphene, each partially covering the same flake of α-RuCl3 (see
Inspired by traditional modulation doping that employs an intermediate insulating layer to separate donors/acceptors from the charged layer, a third device design was explored. Device D3 contains three regions of bare mlg, mlg and α-RuCl3 in direct contact, as well as mlg and RuCl3 separated by ≈3 nm-thick hBN. As the valence band maximum of hBN is closely aligned with the work function of α-RuCl3 (
The G peak maps of devices D1, D2, and D3 all indicate the lateral charge transfer is short, changing abruptly across the α-RuCl3 boundary. This is illustrated via the linecuts in
Similarly crucial is the homogeneity of the induced charge. Given the short lateral extent, regions where the α-RuCl3 is not in good contact with graphene may have little to no induced charge, yielding a Raman spectra with both shifted and unshifted peaks, as shown by the spectra at three different locations of device D1 (inset of
To quantify the uniformity, homogeneity was defined to be 100% when only a shifted G peak is present, whereas 0% homogeneous regions exhibit shifted and unshifted peaks with equal spectral weight. device homogeneity was then quantified as (2×Inorm)−1, where Inorm is the intensity of the shifted G peak normalized by the sum of intensities of both peaks. The map of sample D1's homogeneity, shown in
These observations resolve outstanding issues in mlg/α-RuCl3 devices. Specifically, the appearance of a Dirac point near zero gate voltage in otherwise extremely conductive and highly hole-doped graphene (
Lastly, the range of charge transfer in α-RuCl3 heterostructures was explained via the relative twist angle. Devices disclosed herein and previous reports indicate a large variation in hole densities over 2-4×1013 cm−2 far greater than the spread within a single device (δp≈1-5×1012 cm−2) (
In some embodiments, similar effects emerge from a low work function material acting as a 2D crystalline donor. As such, modulation doping can be introduced into two-dimensional heterostructures with far reaching implications. For example, a 2D material can be uniformly or locally charged by controlling the regions over which it touches a crystalline acceptor or donor. This enables a new regime of two-dimensional plasmonics, improved electrical transparency of contacts by locally doping the contacted layer, and the creation of lateral p-n junctions. Such devices require expanding the doping to a wider set of two-dimensional materials, as indicated by
Ultra-Sharp Lateral p-n Junctions in Modulation-Doped Graphene
Ideal p-n junctions in graphene with a step-function change in carrier density underlie the physics of Klein tunneling, negative refraction leading to Veselago lensing, guiding of plasmons and snake states, and may enable controlled anisotropy of the band velocity or novel electron-optical devices based on transformation optics or the ability to focus electron beams. In practice, p-n junctions defined by electrostatic gating are far from this ideal, with the change in carrier density taking place over 50-100 nm due to fringe electric fields from the edges of the metallic gates. Charge carriers traversing such broad interfaces are collimated since the likelihood of transmission is exponentially suppressed for carriers incident at increasing angles from the normal. Despite this, p-n (also p-n-p) junctions enable observation of Klein tunneling, magnetic focusing in ballistic graphene, and can be used to create a switch based on the reflection of charge carriers. But to achieve the transmission over a wide range of impact angles needed to clearly observe and apply Veselago lensing requires junctions closer to the ideal case. Modulation-doping of graphene was employed by proximity to α-RuCl3, and a well-defined boundary to the doped region via a cleaved edge of the α-RuCl3 flake, to create ultra-sharp junctions, demonstrated with evidence from electronic transport, scanning tunneling probes of the spatial variation of the charge density in doped graphene, and first principles calculations.
When the layered Mott insulator alpha-ruthenium(III) chloride (α-RuCl3) is placed in direct contact with graphene, it accepts approximately 4×1013 cm−2 electrons, leaving the graphene strongly hole-doped. This is several times larger than can be achieved with typical back-gating for graphene-on-silicon oxide devices. If an insulating spacer is introduced between the two materials, the charge transfer persists but is weakened commensurate with the setback of α-RuCl3 from graphene. Meanwhile the carrier mobility of the graphene remains remarkably high when in direct contact with α-RuCl3, reaching 5000 cm2/Vs, and can further increase with the use of an insulating spacer. This behavior is analogous to modulation doping of conventional two-dimensional electron gases, except that here the acceptor material is a crystalline sheet rather than randomly distributed impurity atoms. Because the charge transfer takes place only where α-RuCl3 overlaps graphene, the spatial distribution of the hole-doping can be readily controlled using van der Waals device fabrication techniques. Optical experiments on a first generation of devices having regions of intrinsic graphene abutting regions that were modulation-doped by α-RuCl3 placed an upper bound of 65 nm on the lateral extent of the charge transfer into the graphene sheet away from the edge of α-RuCl3 flakes, while recent infrared near-field and scanning tunneling measurements indicate the boundary can be just nanometers in width. Patterned charge-doping by α-RuCl3 thus appears to be a viable route toward ultra-sharp p-n junctions in graphene.
Here modulation doping by α-RuCl3 was used to differentially charge-dope two regions of graphene. With additional control of the carrier densities via global top and back gates, transport in the bipolar regime can be explored. Resistance measured across the junction was consistent with a very narrow and highly transmissive p-n junction at the interface of the intrinsic and doped regions. In two such devices, very different junction widths are observed which correlates with the presence or lack of a cleaved crystalline edge of the dopant α-RuCl3 flake. Low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) were also used to explore devices where a graphene sheet is either directly in contact with α-RuCl3 or separated from it by thin flakes of hexagonal boron nitride (hBN). A sharp change was observed in the charge doping of the graphene over a sub-10 nm length scale across step edges in the insulating hBN spacer. Finally, density functional theory (DFT) calculations were performed to reveal that the hole-doping of graphene due to electron transfer to α-RuCl3 falls off rapidly over several graphene lattice constants away from the α-RuCl3 edge.
Electronic Transport. The electronic transport in two graphene devices containing lateral p-n junctions was investigated. In both, half the graphene sheet is intrinsic while the other half is modulation-doped by an α-RuCl3 flake through a thin insulating layer. Device D1 has a ≈1.5-nm-thick AlOx film between the graphene and α-RuCl3, while device D2 has a 2-nm-thick flake of hBN as the spacer.
Simultaneous four-terminal resistance measurements at T=4 K of the g and mod side of device D1 are shown in
The width of a graphene p-n junction can be determined by its contribution to the total device resistance. Charge carriers incident on a p-n junction in graphene obey an electronic analog of Snell's law at an interface of right- and left-handed optical materials: the momentum along the junction, ky=kF sin θ, is conserved, but the momentum kx normal to the junction changes sign along with the sign of the carriers in passing from one side to the other with the end result being a negative refraction. Here kF is the Fermi momentum and θ is the carrier angle of incidence on the junction measured from the normal. Across an ideal abrupt junction, carriers transmit to a final state with probability T(θ)=cos2 θ due to pseudospin conservation. In real devices there is always a density gradient from p-to n-type over some characteristic junction width d, analogous to the depletion region of a classical doped-Si p-n junction but having a different origin. Although there is no band gap in graphene, an effective gap to transmission arises when kx(x)=√{square root over ((E(x)/ℏF)2−ky2)}, becomes imaginary, where E(x)=ℏFkF is the position-dependent energy of the graphene Dirac point across the junction, and νF≈106 m/s is the Fermi velocity. This is depicted schematically in
T(θ,d)=cos2θe−πk
In turn, the reduced transmission leads to a finite resistance that has both ballistic and diffusive contributions, Rp-n=Rbal+Rdif, with magnitudes depending primarily on the carrier mean free path relative to the junction width but also on many-body effects. Experimentally, p-n junction resistances have ranged from a few hundred ohms in graphene-on-oxide junctions to 100Ω in hBN-encapsulated junctions.
In the following, the width of lateral p-n junctions was extracted in two devices as follows, illustrating the procedure by closely analyzing transport in device D1. First,
Next the resistance is isolated, Rp-n, of the p-n junction itself, starting with line cuts of Rjn along lines of equal carrier density and same sign (ng=nmod>0, yielding Rjnn-n) or opposite sign (ng=−nmod>0, Rjnn-p). These are plotted together in
It remains to subtract this asymmetric part of the sheet resistance to finally obtain the resistance of the p-n junction: Rp-n=Rjnodd−(cg×Rgodd+cmod Rmododd), where ci are scaling factors appropriate for the geometry of the contacts.
These results are compared to theoretical predictions for electronic transport through p-n junctions in disordered graphene, and find evidence for an ultra-sharp junction having width d≤10 nm in device D2. In the theory, transport across the junction is a sum of ballistic and diffusive contributions; which term is dominant depends on the ratio of the carrier density gradient across the interface to the impurity density, β=n′/n3/2imp, where nimp is determined from the carrier mobility μ by nimp=el(hμ). For β»1 (<<1) the junction transport is predominantly ballistic (diffusive). To calculate β in these devices, the density gradient is estimated for balanced junctions as n′=2|n|/=d, where n is the density of electrons or holes on both sides of the interface; and using the experimentally measured mobilities to find nimp, both devices easily satisfy β»1 for any value of d below 300 nm. Accordingly, the experimentally-determined junction resistance Rp-n is compared to the predicted resistance of a ballistic junction, Rbal=c(h/e2)/(α1/6n′1/3W). Here W is the device width, α≈0.3 (0.5) is the graphene fine structure constant for the device with AlOx (hBN) spacer, and c≈1 captures the (α-dependent) effect of many-body effects in the ballistic junction. In this equation the only remaining free parameter is the junction width d in the density gradient n′, so d is varied to produce curves of Rbal as a function of carrier density that best fit the data. The results are plotted in
Device D2 is thus found to have an ultra-sharp, sub-10-nm junction, while D1 has a much wider ˜100 nm junction. This result is surprising because the two devices are so different. Both have insulating spacers of approximately the same thickness, with modulation-doping values that differ by only a factor of two. The differing mobilities are unlikely to be the culprit, since the β parameter indicates transport across the junction is firmly in the ballistic regime. The interface in device D1 is angled at 22° so the junction appears wider, but only by a factor of 1/cos(22°)≈1.08. Ultimately, inspection of the constituent flakes of the devices offers a clear resolution: in D1, the edge of the α-RuCl3 flake at the boundary between the intrinsic and modulation-doped regions is slightly curved and has no obvious relation to its crystalline axes. In contrast, in D2 the relevant edge is visually straight and makes an angle of ≈119° with another portion of the flake just outside where it contacts the graphene. This indicates the boundary in D2 is a cleaved crystalline edge, but in D1 is likely to be rough with various facets along the edge.
In the embodiments of the present disclosure, charge transfer takes place when a 2D active conducting material and a 2D charge transfer source material overlap such that the 2D active conducting material overlaps the 2D charge transfer source material including at least one edge of the 2D charge transfer source material. In some embodiments, the edge of the 2D charge transfer material may have a straight or non-straight edge, and/or may have a cleaved or non-cleaved edge. As disclosed herein, a straight edge helps define the spatial boundary of the charge transfer such that the boundary can be quite sharp (e.g., with a straight, cleaved edge). The boundary is defined by the charge transfer region's edge (i.e., the length over which the charge transfer effect ceases to operate), with an ultra-sharp boundary being on the order of from about 1 nm to about 10 nm wide. In embodiments without such a cleaved edge, the charge transfer region's edge may have a length of from about 100 nm to about 200 nm.
Scanning Tunneling Microscopy. Scanning tunneling microscopy and spectroscopy measurements at T=4.8 K were used to further study the spatial variation of the Dirac point across differentially-doped regions, in two other devices, D3 and D4, both composed of overlapping flakes of graphene, hBN, and α-RuCl3on a SiO2/p-Si substrate.
When an hBN spacer layer of variable thickness is inserted between the graphene and α-RuCl3 as shown in
Density Functional Theory Calculations. To understand both the lateral and vertical spatial distribution of the charge transfer due to the modulation-doping of graphene by α-RuCl3, first principles calculations were performed of a monolayer-thick α-RuCl3 ribbon on graphene as shown in
With the two materials in close proximity, a new charge density distribution develops which was illustrated by calculating the difference with respect to the intrinsic materials, Δρ=ραR/g−ραR−ρg. Charge accumulates in the α-RuCl3 ribbon with a concomitant depletion in the graphene, as shown in
The equilibrium height of the α-RuCl3 above the graphene, s0=3.31 Å, is defined as the average distance between the C atoms in graphene and the graphene-facing Cl atoms in α-RuCl3, shown in
Finally, the characteristic length scale can be estimated over which the charge transfer decays away outside the ribbon by fitting the decrease of the charge density peaks around the C atoms, visible in
By three distinct and complementary approaches, sub-10 nm changes are demonstrated in the charge doping of graphene near the edge of an α-RuCl3 flake, or across a step height in an insulating barrier between these two materials. To achieve such a sharp boundary, it appears crucial to use a cleaved, crystalline edge of α-RuCl3 to clearly define an ultra-sharp boundary of the modulation-doped region. p-n junctions defined this way are narrow enough to enable observation of electron-optical effects such as Veselago lensing and other useful devices based on electron refraction or reflection.
Ultimate junction width limits. The decay length extracted from the present DFT calculations—which assume a crystalline armchair edge to the α-RuCl3 ribbon—is just a few times greater than the graphene/α-RuCl3 separation distance. In device D2, the insulating spacer layer is 2 nm thick, so that a junction width of ≈5 nm is expected, and a line is extrapolated from the data in
In the pursuit of nearly-ideal p-n junctions, recent work has identified three outstanding hurdles: finite junction width, junction interface roughness, and nonlinearity and asymmetry of the doping profiles around the junction. The present disclosure describes elimination of all of these problems (with the possible exception of nonlinear screening effects) by using the cleaved crystalline edge of a flake of α-RuCl3 to define a sharp edge between differentially-doped regions of graphene. This advance enables observations and applications based on Veselago lensing and other electron-optic phenomena.
Materials and Methods
Raman scattering and photoluminescence. A WITec system inside Ar environment glovebox has automatic mapping with a 532 nm laser, 1 μm spot size and 1800 g/mm grating is used in this experiment. The laser power is 300 μW, the integration time is 25 s and the step size is 0.3 μm for all Raman maps. Due to additional disorder, the CVD graphene signal was weaker and thus 300s integration time was employed. The EuS and EuS/RuCl3 measurement are performed with 100 μW with 300s integration time.
Device fabrication and transport. Devices were fabricated using a dry van der Waals stacking technique to sequentially pick up and stack layers of graphene, hexagonal boron nitride, α-RuCl3, and CrCl3, using either poly(bisphenol a) carbonate (PC) or polypropylene carbonate (PPC) films as the adhesive layer. CrCl3 flakes were only exposed to a N2 atmosphere, with the rest assembled in air. Cr/Au top gate and edge contacts were patterned by standard e-beam lithography. Transport measurements were carried out in a Quantum Design Physical Properties Measurement System (PPMS).
Material Growth. Single crystals of α-RuCl3 were grown using a vapor transport technique from phase pure commercial α-RuCl3 powder. Single crystals of CrCl3 are grown by recrystallizing CrCl3 powder in an evacuated quartz tube with temperature gradient 650-550° C. for one week. EuS (10 nm) was deposited at room-temperature at 10−8 torr, on freshly cleaved RuCl3 surface, using an e-beam evaporation technique. Monolayer CVD graphene was grown on a copper foil via low pressure CVD (as described herein elsewhere).
Peak fitting. To accurately represent the phonon frequencies and amplitudes, the Raman spectra was fit with the Voigt function
I(ω,σ,Γ,A)=∫−∞∞G(ω′,σ)L(ω−ω′,Γ,A)dω′
which is a convolution of a Gaussian and a Lorentzian. Here the Lorentzian represents a phonon mode and a Gaussian was used to account for the instrumental resolution. The Gaussian width σ=1 cm−1 is determined by the central Rayleigh peak. Γ is the phonon width and A is phonon amplitude.
For the mlg G peak spectra, a fit range of 1550-1650 cm−1 was chosen, using a constant background and three-Voigt peaks to capture the broad background that appears in this energy range from the α-RuCl3 itself, as well as the presence of both shifted and unshifted G-peaks. For the mlg and blg 2D peak spectra, fit was from 2650-2750 cm−1, with a constant background and two-Voigt peaks, for both the shifted and unshifted 2D peaks. For the blg 2D peak, four-Voigt peaks with a constant background were used to capture the four characteristic blg 2D peaks (1A, 1B, 2A, 2B). Raman shift distributions were plotted against 2D1 Å peak, and this peak was used to calculate blg carrier density, as this peak follows the same trend as the mlg 2D peak.
Determination of strain and doping from Raman. Raman phonon frequencies in graphene are sensitive to both doping and strain. However, these two effects can be separated via correlation decoupling analysis of the G and 2D frequencies. According to
lD*sin(θ1)+lS*sin(θ2)=Δω2D
lD*cos(θ1)+lS*cos(θ2)=Δω2G
By inverting this equation, the measure (ΔωG, ω2D) can determine the strain and Fermi level. A downward (upward) projected vector along the strain line
To determine the precise doping level, the results of three independent measurements and a theory calculation shown in
Here, α is the linear fitting of ΔωG and EF for mlg. From the average of four reference electric gating studies, α=−45.7 eV/cm−1. Therefore, after converting EF to carrier density the expression below was used to get the carrier densities of monolayer graphene. As for strain, it also linearly depends on the G peak frequency shift.
Here, νF is the Fermi velocity, s is the percent of strain. For uniaxial strain s is −23.5 cm−1/% and for biaxial strains is −69.1 cm−1/%.
In the bilayer graphene case, the carrier density to G peak shift relationship is slightly different from monolayer graphene. To convert Raman peak shifts in bilayer graphene to doping levels, the peak shift values were projected along the doping axis, as with monolayer graphene, then utilized reference results (see
Theory DFT. The DFT calculations are performed within the generalized gradient approximation (GGA) using Perdew-Burke-Ernzerhof (PBE) functional implemented in Vienna Ab initio Simulation Package (VASP).12 A plane-wave basis set with a kinetic energy cutoff of 450 eV, and a 4×3×1 k-point sampling grid is adopted to a heterostructure supercell with cell constant of 12.03 Å. The geometric structure of heterostructures are relaxed by fixing hBN and graphene layers to the α-RuCl3 lattice constant (6.02 Å) with fully relaxed force of α-RuCl3. This relaxation scheme better mimics the band alignment and charge transfer since the work function of graphene is not sensitive to strain and the strain effect on the wide gap of hBN is small. The proper super cell of α-RuCl3 (2×2×1) and for hBN (5×4×1) and graphene (5×4×1) are used to reduce the stress induced by the lattice mismatch between materials while balancing the computational burden. The vacuum distance is set to be around 18 Å along the z-direction to avoid spurious interactions. The vdW interaction is included by the DFT-D2 method and spin orbit coupling (SOC) is always considered. The choice of Hubbard U=2.4 eV and Hund J=0.4 V for Ru3+ ions is based on previous studies.
Theory: MINT. The ab initio MINT calculations were carried out within the total-energy plane wave density-functional pseudopotential approach, using Perdew-Burke-Ernzerhof generalized gradient approximation functionals and optimized norm-conserving Vanderbilt pseudopotentials in the SG15 family. Plane wave basis sets with energy cutoffs of Hartree were used to expand the electronic wave functions. Fully periodic boundary conditions and a single unit cell of α-RuCl3 with a 6×4×1 k-point mesh were used to sample the Brillouin zone. Electronic minimizations were carried out using the analytically continued functional approach starting with an LCAO initial guess within the DFT++ formalism, as implemented in the open-source code JDFT×20 using direct minimization via the conjugate gradients algorithm. All unit cells were constructed to be inversion symmetric about z=0 with a distance of ≈60 Bohr between periodic images of the α-RuCl3 surface, using Coulomb truncation to prevent image interaction.
Evidence for charge transfer in MBE grown EuS. EuS is a magnetic semiconductor with a low work function (3.3 eV) than is routinely used in various spintronic and proximity heterostructures. It has a ferromagnet transition temperature of 13K. A 10 nm EuS film grown atop of bulk α-RuCl3 ensured both components were intact by SQUID measurements. As shown in
Consistent with α-RuCl3 significantly doping EuS, the Raman results in (
Evidence for charge transfer into CVD grown WSe2. Monolayer WSe2 has a direct band gap and strong exciton binding energy. As the relatively sharp photoluminescence (PL) emission can be sensitive to the chemical potential. In particular for neutral WSe2 the exciton is observed, whereas when doped it forms trions. Thus the exciton emission can be used to detect the carrier type (doping level) and density in monolayer WSe2. To investigate this, a CVD grown and transferred α-RuCl3 flake was employed on top. The PL from the bare WSe2 peaks at 1.65 eV, where the region covered by α-RuCl3 has significantly narrowed and shifts to 1.665 eV (
Charge transfer into CVD graphene. The charge transfer between α-RuCl3 and graphene also can be realized in CVD graphene/RuCl3 heterostructures, crucial for eventual scale-up. To demonstrate this, thin α-RuCl3 flakes were gently exfoliated on top of a CVD graphene film on SiO2/Si substrate (as described herein elsewhere). As shown in
Optoelectronic response of graphene/α-RuCl3heterostructures. Graphene can exhibit both photothermal and photovoltaic responses. The former is driven by electron temperature gradient and the latter is generated by built-in electric fields. Both can be enhanced at high carrier densities but suppressed at the charge neutral point. As such the photovoltage can be useful in detecting charge in homogeneity and the presence of homojunctions in graphene. As shown in
Electronic transport.
Interference correction. Raman scattering can be enhanced or suppressed by the interference from thin layers in the heterostructure. The change in the Raman response from interference can be calculated by applying Fresnel's Law to the thin films and summing up the contribution of different layers. As described herein, hole doping in graphene is detected via graphene G peak shift. Thus, the wavenumber dependent enhancement contribution from α-RuCl3 to graphene layer becomes important. Here, via dividing the mlg/RuCl3 response with the enhancement factor from α-RuCl3, one can get the not-enhanced spectrum (
Range of doping in graphene/α-RuCl3heterostructures. A crucial aspect of the heterostructures presented is the overall uniformity. One measure presented is the presence of neutral puddles inside doped regions. A second is the range of resulting doped values. To access the range of induced dopings, for each device a histogram was made of the local doping level at each point in the Raman maps. These are shown in (
Doping spatial resolution. In the homogeneity map (
d0 from A0, where d0=√{square root over (2*A0/π)}=67 nm
CVD Growth and Fabrication. The copper foil (Alfa Aesar) was pre-treated in Ni etchant (Transene) to remove any coatings or oxide layers from its surface. The tube furnace was evacuated to a read pressure of 200 mTorr with a constant flow of H2 (10 sccm). Prior to growth, the foil was annealed at 1010° C. (ramp rate 25° C./min) for 35 min. Growth was done at 1010° C. with 68 sccm of H2 and 3.5 sccm of CH4 for 15 min. CVD graphene was removed from its copper film by applying a polymethyl methacrylate (PMMA) adhesion layer, followed by removal of the copper with Ni etchant for 2 h at 60° C. The remaining PMMA/graphene structure was washed in water twice for 60 s, and after transfer to Si/SiO2 the PMMA was dissolved in acetone vapors followed by isopropanol alcohol (IPA) and baked at 300° C. for 8 h in vacuum prior to stacking α-RuCl3 on top.
Sample preparation and device fabrication. Graphene, hexagonal boron nitride (hBN), and α-RuCl3 flakes were isolated via mechanical exfoliation. Atomic force microscopy was used to con rm the thickness of the flakes used in each device. Devices D1, D2, D3, and D4 were fabricated using a dry van der Waals stacking technique to pick up and stack layers of graphene, hBN, and α-RuCl3 using an adhesive layer of polypropylene carbonate (PPC) in D1 and poly(bisphenol a) carbonate (PC) in D2, D3, and D4. The aluminum oxide layer in D1 was grown by electron beam evaporation of 1.5 nm of aluminum that was subsequently oxidized. The top gates and contacts used were patterned by electron beam lithography in D1 and photolithography in D2. Contacts and top gates were metallized by thermal evaporation of Cr/Au leads. Devices were etched using XeF2 gas at a chamber pressure of 1500 mTorr for 15 s.
Electronic transport measurements. Electronic transport, including Hall data, in devices D1 and D2 were taken at 4 K in a variable-temperature cryostat using standard low-frequency measurement techniques.
Scanning tunneling measurements. Completed devices were annealed in UHV at 400° C. for several hours before being transferred into the STM chamber. The STM measurements were conducted in a Createc LT-STM with a vacuum better than 1×10−10 mbar at 4.8 K. Electrochemically etched tungsten tips used in the experiments were calibrated by measuring the surface state of Au(111) crystal before all measurements. dI=dVS spectra were acquired with standard locking technique by applying a 704 Hz ac modulation to the sample bias, with a setpoint of I=1 nA and a 10 mV excitation. Topographic data was acquired with a setpoint current of I=10-20 pA and a 10 mV ac excitation. The STM topography images were plotted with WS×M.
Density functional theory calculations. DFT structural relaxations of the interface were performed with the projector augmented wave method using the Vienna ab initio simulation package (VASP), using the generalized gradient approximation (GGA) for the exchange-correlation functional. A correction due to van der Waals forces are included through the DFT-D2 scheme of Grimme. The lattice parameter a is chosen in such a way that the graphene layer is not strained (i.e., C—C bond-length remains 1.42 Å after relaxation). A plane-wave cutoff of 600 eV is used for all the geometries with k-point sampling of 1×3×1. For all geometrical optimizations, Ru atoms are considered to be in ferromagnetic configuration. In order to calculate the amount of charge transfer between the layers, Bader analysis of wavefunctions was used obtained from VASP calculations. Correlation effects and spin-orbit coupling in this heterostructure system are not expected to affect the value of the charge transfer as observed in the commensurate bilayer case.
Definitions and methods described herein are provided to better define the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. Unless otherwise noted, terms are to be understood according to conventional usage by those of ordinary skill in the relevant art.
In some embodiments, numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth, used to describe and claim certain embodiments of the present disclosure are to be understood as being modified in some instances by the term “about.” In some embodiments, the term “about” is used to indicate that a value includes the standard deviation of the mean for the device or method being employed to determine the value. In some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters are be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the present disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the present disclosure may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.
In some embodiments, the terms “a” and “an” and “the” and similar references used in the context of describing a particular embodiment (especially in the context of certain of the following claims) are construed to cover both the singular and the plural, unless specifically noted otherwise. In some embodiments, the term “or” as used herein, including the claims, is used to mean “and/or” unless explicitly indicated to refer to alternatives only or to refer to the alternatives that are mutually exclusive.
The terms “comprise,” “have” and “include” are open-ended linking verbs. Any forms or tenses of one or more of these verbs, such as “comprises,” “comprising,” “has,” “having,” “includes” and “including,” are also open-ended. For example, any method that “comprises,” “has” or “includes” one or more steps is not limited to possessing only those one or more steps and may also cover other unlisted steps. Similarly, any composition or device that “comprises,” “has” or “includes” one or more features is not limited to possessing only those one or more features and may cover other unlisted features.
All methods described herein are performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the present disclosure and does not pose a limitation on the scope of the present disclosure otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the present disclosure.
Groupings of alternative elements or embodiments of the present disclosure disclosed herein are not to be construed as limitations. Each group member is referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group are included in, or deleted from, a group for reasons of convenience or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.
To facilitate the understanding of the embodiments described herein, a number of terms are defined below. The terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present disclosure. Terms such as “a,” “an,” and “the” are not intended to refer to only a singular entity, but rather include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the disclosure, but their usage does not delimit the disclosure, except as outlined in the claims.
All of the compositions and/or methods disclosed and claimed herein may be made and/or executed without undue experimentation in light of the present disclosure. While the compositions and methods of this disclosure have been described in terms of the embodiments included herein, it will be apparent to those of ordinary skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit, and scope of the disclosure. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the disclosure as defined by the appended claims.
This written description uses examples to disclose the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
Claims
1. A charge-transfer controlled 2D device comprising:
- a 2D active conducting material;
- a 2D charge transfer source material; and
- at least one overlapping portion wherein the 2D active conducting material overlaps the 2D charge transfer source material including at least one edge of the 2D charge transfer source material.
2. The device of claim 1, wherein the at least one overlapping portion comprises the 2D active conducting material being in direct contact with the 2D charge transfer source material.
3. The device of claim 1, further comprising an insulating layer such that the at least one overlapping portion comprises the insulating layer disposed between the 2D active conducting material and the 2D charge transfer source material.
4. The device of claim 1, wherein the 2D active conducting material is selected from graphene, WSe2, and EuS.
5. The device of claim 1, wherein the at least one edge of the 2D charge transfer source material is a straight cleaved edge.
6. The device of claim 1, wherein the 2D charge transfer source material comprises at least one of a crystalline material and an exfoliated material.
7. The device of claim 6, wherein the 2D charge transfer source material is selected from alpha-RuCl3 and tungsten oxyselenide.
8. The device of claim 3, wherein the insulating layer is selected from hexagonal boron nitride and AlOx.
9. A method for controlling charge transfer in 2D materials, the method comprising:
- providing a 2D active conducting material;
- providing a 2D charge transfer source material; and
- positioning the 2D active conducting material to overlap at least one portion of the 2D charge transfer source material including at least one edge of the 2D charge transfer source material.
10. The method of claim 9, wherein the positioning comprises overlapping the 2D active conducting material in direct contact with the 2D charge transfer source material.
11. The method of claim 9, further comprising providing an insulating layer on a surface of the 2D charge transfer source material, and wherein the positioning comprises overlapping the 2D active conducting material in direct contact with the insulating layer on the surface of the 2D charge transfer source material.
12. The method of claim 9, wherein the 2D active conducting material is selected from graphene, WSe2, and EuS.
13. The method of claim 9, wherein the at least one edge of the 2D charge transfer source material is a straight cleaved edge.
14. The method of claim 9, wherein the 2D charge transfer source material comprises at least one of a crystalline material and an exfoliated material.
15. The method of claim 14, wherein the 2D charge transfer source material is selected from alpha-RuCl3 and tungsten oxyselenide.
16. The method of claim 11, wherein the insulating layer is selected from hexagonal boron nitride and AlOx.
17. A charge-transfer controlled 2D system comprising:
- a top gate;
- a charge-transfer controlled 2D device including a 2D active conducting material, a 2D charge transfer source material, and at least one overlapping portion wherein the 2D active conducting material overlaps the 2D charge transfer source material including at least one edge of the 2D charge transfer source material; and
- a bottom gate.
18. The system of claim 17, wherein:
- the top gate is selected from Cr/Au and hexagonal boron nitride-supported Cr/Au;
- the 2D active conducting material is selected from graphene, WSe2, and EuS;
- the 2D charge transfer source material is crystalline, exfoliated alpha-RuCl3; and
- the bottom gate comprises SiO2/p-Si.
19. The system of claim 17, wherein the at least one overlapping portion further comprises an insulating layer disposed between the 2D active conducting material and the 2D charge transfer source material.
20. The system of claim 19, wherein the insulating layer is selected from hexagonal boron nitride and AlOx.
Type: Application
Filed: Jul 12, 2022
Publication Date: Jan 12, 2023
Inventors: Erik Henriksen (St. Louis, MO), Jesse Balgley (St. Louis, MO), Kenneth S. Burch (Newton, MA), Yiping Wang (Brighton, MA)
Application Number: 17/811,934